# code 1-2 P18
# 定义一个函数
# todo:注释每一句的意思
def compute_gpa(grades, points=None):
    if points is None:
        points = {'A+': 4.0, 'A': 4.0, 'A-': 3.67, 'B+': 3.33, 'B': 3.0, 'B-': 2.67,
                  'C+': 2.33, 'C': 2.0, 'C-': 1.67, 'D+': 1.33, 'D': 1.0, 'F': 0.0
                  }
    num_courses = 0
    total_points = 0
    for g in grades:
        if g in points:
            num_courses += 1
            total_points += points[g]
    return total_points / num_courses
